1. Field of the Invention
The present invention relates to the design and manufacture of thin film photovoltaic cells and, more particularly, to commercially practical techniques for forming a protective backcap on a photovoltaic panel such that high panel efficiency stability is maintained during the life of a photovoltaic panel.
2. Description of the Background
Photovoltaic technology has been widely used for years in every industrialized nation. Common devices, such as watches worn by thousands of people, are powered by sunlight, and buoys in numerous shipping channels throughout the world are lighted by an array of photovoltaic crystal slices electrically connected in series. Within the past decade, photovoltaic technology has slowly been applied to the manufacture of photovoltaic panels to drive increasing numbers of remotely located electrically powered devices, such as refrigerators for the storage of medicine, pumps for irrigation, and telecommunication stations. Visionaries, however, plan for the large scale manufacture of photovoltaic panels in sufficient quantities to power entire villages worldwide. If such an objective is to be achieved, the manufacturing cost for photovoltaic panels should be low, the panel conversion efficiency should be relatively high, and the photovoltaic panels should exhibit high conversion efficiency stability during the life of the panels.
Those skilled in the art have long recognized that low manufacturing cost for photovoltaic panels can be achieved through the use of thin film photovoltaic technology, especially when that technology is applied to large scale manufacturing operations. Relatively high photovoltaic conversion efficiencies beyond the order of 7% and 8% can now be obtained in commercial production operations. While others have addressed the problem of protecting thin film photovoltaic cells from adverse exterior environmental elements, such as water or reactive gases, the present invention is uniquely able to tie these prior achievements together by providing a low cost, commercially practical technique for protecting the photovoltaic cells from both interior and exterior elements. The invention also enables the manufacturer to subject the cells to a selected gas during processing and during the life of the panels, thereby achieving the desired high conversion efficiency and long panel life.
Techniques relating to the manufacture of photovoltaic cells have been devised for decades. Concepts developed in the 1960's are disclosed, for example, in U.S. Pat. Nos. 3,520,732 and 3,568,306. Thin film photovoltaic cells may be easily formed on a vitreous substrate, such as glass, as taught by U.S. Pat. Nos. 4,086,101, 4,362,896, 4,412,091, 4,265,933. Photovoltaic cells formed on a glass substrate may be connected electrically in series, as taught by U.S. Pat. No. 4,243,432. A plurality of electrically connected cells thus form a solar panel, and a number of solar panels may be mounted in a module as taught by U.S. Pat. No. 4,233,085. Large scale manufacturing techniques for forming such panels are disclosed in U.S. Pat. No. 3,880,633, 4,228,570, 4,307,681, 4,239,809, 4,492,605. The bottom electrode layer for each of the plurality of thin film photovoltaic cells may be formed from various materials and by various techniques, such as those disclosed in U.S. Pat. No. 4,401,291, 4,178,395, 4,256,513, 4,362,896.
A photovoltaic cell having a cadmium telluride layer is disclosed in U.S. Pat. No. 4,568,792. Techniques for reducing manufacturing costs for forming such a cell, which includes a polycrystalline monolayer, are disclosed in U.S. Pat. No. 4,735,909. U.S. Pat. No. 4,578,526 discloses a thin film photovoltaic cell on a glass substrate, with the photovoltaic unit including a back plate which is adhered with a resin layer to protect the photovoltaic cell.
U.S. Pat. No. 4,633,032 discloses another type of solar cell on a glass substrate. The solar cells are arranged in a rather complex "package configuration", and the frame of the package houses a desiccant. This "package configuration", which significantly increases the material costs and manufacturing costs for the solar panels, also includes a synthetic encapsulating resin which adheres to the back plate as well as the glass substrate, and provides the desired stress relief between components of the complex frame. U.S. Pat. No. 4,705,911 discloses a solar cell module which includes a CdS/CdTe heterojunction, with the photovoltaic cell also being formed on a vitreous substrate. This patent discloses an oxygen releasing agent spaced adjacent the vitreous substrate for minimizing the reduction and its associated decrease in conversion efficiency, thereby increasing the useful life of the photovoltaic cell.
Prior art techniques for forming a protective back plate or backcap on a photovoltaic cell are expensive to manufacture and/or do not adequately protect the photovoltaic cell from exterior environmental elements or exhibit incompatabilities with the chemically sensitive film layers. The present invention overcomes the problems and disadvantages of the prior art, and enables the manufacture of photovoltaic cells having a relatively high conversion efficiency and a low cost. Thin film photovoltaic technology may thus be applied in commercial applications to achieve the necessary high cell conversion efficiency stability required by purchasers of such photovoltaic panels.